This invention relates to printing press apparatus and methods of operation thereof and more particularly, to a blanket cylinder assembly and method of making same to reduce gap disturbances over a range of high speeds to a level beneath a threshold level at which streaking is caused during printing operations.
The performance boundaries of web-fed rotary printing presses have traditionally been limited by the phenomenon of "streaking" in the form of partial or complete ink discontinuities which extend along one or more lines parallel to one another and transverse to the direction of travel of paper. It is known that this phenomenon is the result of transient vibrations of the printing cylinders induced by the repetitive passage of surface discontinuities through the line of contact between coacting cylinders. Such discontinuities are present in lithographic process printing presses as a consequence of the need for removable, image-carrying plates and for removable, resilient blankets used for image offsetting to the paper or for impression support behind the paper when printing is done directly from the plate. Various mechanisms are known which secure the ends of plates and blankets to the cylinders that require some space for insertion and removal of the ends which disallows a continuous surface around the cylinder circumference.
In contrast, rotogravure presses operate with the image engraved directly into the cylinder surface. This permits a continuous surface, and thus rotogravure presses do not exhibit the streaking phenomenon. Unfortunately, when the image has to be changed, the entire cylinder must be removed from the press.
A typical printing press of the general type to which this invention relates will exhibit an increasing tendency to produce streaked printing as the rate of cylinder rotation, or press speed, increases. Thus some maximum operating speed is established at which streaking is not observable or not intense enough to cause rejection of the printed product. Observation of the behavior of such a press has lead others to the conclude that streaking is a monotonic function of press speed. Based on this conclusion, certain actions have been taken by others to provide a greater range of acceptable press performance as it is judged in regard to streaking.
Attempts have been made to reduce the severity of the disturbance created by the passage of the cylinder discontinuity. Kirkus teaches in U.S. Pat. No. 3,395,638 that this can be accomplished by gradually reducing the cylinder radius as the discontinuity is circumferentially approached from either direction. This has the effect of reducing the time dependent force derivatives that contribute to the imposed disturbance. An expedient method used to emulate this effect is to "feather" the sheets of paper that are placed between the blanket cylinder body and the blanket to obtain the correct overall dimension for printing. Feathering is the process of placing several such sheets of paper on the cylinder which are cut to different lengths so the effective radius of the blanket cylinder is reduced in the vicinity of the discontinuity location. Bartlett teaches in U.S. Pat. No. 4,466,349 that designing the cylinder so that the line of the discontinuity is at an angle of skew relative to the axis of cylinder rotation will reduce the disturbing effect by allowing the discontinuity to pass through the line of contact progressively from one end of the cylinder to the other instead of along the entire cylinder at one time.
These two approaches to disturbance magnitude reduction have not found widespread use for two reasons. One is that the attempt to reduce the pressure gradient in the vicinity of the discontinuity also necessarily reduces the pressure available to affect ink transfer and therefore places a limit on the cylinder circumference which can actually be used for printing. The second reason is that manufacturing variable radius cylinders and skewed discontinuities is more complicated and thus more costly than manufacturing conventional cylinders. The feathered packing approach adds complexity to press operation and thus increases the variable cost of print production.
Attempts have been made to counter the streaking effect by providing a damping mechanism to more rapidly dissipate the energy imparted to the cylinders by the discontinuity. In U.S. Pat. No. 4,125,073 to the present inventor, an impact damper is incorporated into a cylinder to create a process of momentum transfer which prevents persistent transient oscillation of the cylinder following a disturbance. While such a damper has great advantage, it is difficult to manufacture because of the precise tolerances required for optimal performance and is also subject to wear which reduces its effectiveness over time.
The failure of these attempts to provide a completely satisfactory solution to the streaking problem has led some in the industry to believe that the problem must be solved by eliminating the discontinuity in the cylinder surfaces. This way of thinking implies that any discontinuity, however small, will ultimately produce the streaking phenomenon if the press is run at a high enough speed. Kirkpatrick and Warll in U.S. Pat. No. 3,765,329; Matuschke in U.S. Pat. No. 4,403,549; Banike in U.S. Pat. No. 4,577,560 and Zeller in U.S. Pat. No. 4,742,769 teach methods for complete elimination of discontinuities. However, in applying these methods extreme precision in the gross dimensions of the removable plates and blankets is required if the intention is to make the ends of these elements meet in full contact over the length of the cylinders, but such precision is inconsistent with the normal operating environment of a printing facility. Alternatively, providing means for sealing a residual gap when the ends of the elements cannot be made to meet perfectly complicates the installation and removal processes and thus increases the time and cost associated with preparing a press for operation.